The plastic stomatal development in grasses and its implications in crop improvement

Cereal crops, predominantly belonging to the grass family (Poaceae), are the cornerstone of global agriculture; thus, improving their environmental adaptability has emerged as a pivotal research focus. Stomata, which act as essential conduits mediating gas exchange and water loss in plants, are key targets for genetic modification to enhance crop drought tolerance and water use efficiency (WUE). Unlike the two-celled stomatal complexes of Arabidopsis thaliana, grasses possess four-celled stomata (two dumbbell-shaped guard cells flanked by two subsidiary cells, SCs) with distinct developmental mechanisms, which makes them critical for grasses to adapt to adverse environments. This review summarizes recent advances in understanding such species-specific developmental mechanisms in grasses, with three core emphases: Firstly, grasses possess four-celled stomatal complexes with distinct vein-proximal patterning, and plasticity of stomatal density in response to environmental stimuli (e.g., CO₂, drought stress, temperature, and light) among species; Secondly, functional characterization of key regulators (e.g., BASIC HELIX-LOOP-HELIX (bHLH) transcription factors) governing distinct stomatal developmental stages (e.g., initiation, division, differentiation), and their potential for genetic manipulation to optimize stomatal traits for improved WUE and drought resistance; Lastly, the integration of multi-omics approaches, including single-cell RNA-seq (scRNA-seq), spatial transcriptomics (ST), pan-genomics, genome-wide association studies (GWAS), and artificial intelligence (AI)-driven data mining for accelerating the identification of novel regulators and genotype-phenotype associations in more grass crops. This review provides a comprehensive framework for understanding grass-specific stomatal development and offers actionable targets for precision breeding of drought-resilient cereal crops.